Glaucoma refers to a group of optic neuropathies that are characterized by loss of retinal ganglion cells and atrophy of the optic nerve with resultant visual field loss. The disease is the leading cause of irreversible blindness worldwide and the second leading cause of blindness, behind cataracts. Clinical trials have demonstrated that elevated IOP is a major risk factor for glaucoma and have validated the role of lowering IOP in the management of glaucoma.
Glaucoma is classified according to three parameters: 1) the underlying cause, i.e., primary (idiopathic) or secondary (associated with some other ocular or systemic conditions); 2) the state of the anterior chamber angle, i.e., open angle (open access of the outflowing aqueous humor to trabecular meshwork) or closed angle (narrow angle; the trabecular meshwork is blocked by apposition of the peripheral iris and the cornea); and 3) chronicity, i.e., acute or chronic. Although secondary forms of glaucoma with clear etiologies do exist (e.g., pseudoexfoliation and pigmentary dispersion), the most common form of glaucoma is primary open angle glaucoma (POAG).
OHT is a condition in which IOP is elevated but no glaucomatous findings have been observed (Bell, 2005). The Ocular Hypertension Study demonstrated that patients with OHT have an overall risk of 10% over 5 years of developing glaucoma and that this risk can be cut in half by the institution of medical treatment that reduces IOP. Drug therapies that have proven to be effective for the reduction of intraocular pressure include both agents that decrease aqueous humor production and agents that increase the outflow facility. Such therapies are in general administered by one of two possible routes: topically (direct application to the eye) or orally. However, pharmaceutical ocular anti-hypertension approaches have exhibited various undesirable side effects. For example, miotics such as pilocarpine can cause blurring of vision, headaches, and other negative visual side effects. Systemically administered carbonic anhydrase inhibitors can also cause nausea, dyspepsia, fatigue, and metabolic acidosis. Certain prostaglandins cause hyperemia, ocular itching, and darkening of eyelashes, irises, and periorbital tissues. Further, certain beta-blockers have increasingly become associated with serious pulmonary side-effects attributable to their effects on beta-2 receptors in pulmonary tissue. Sympathomimetics cause tachycardia, arrhythmia and hypertension. Such negative side-effects may lead to decreased patient compliance or to termination of therapy such that normal vision continues to deteriorate. Additionally, there are individuals who simply do not respond well when treated with certain existing glaucoma therapies.